Whenever a lens is positioned in the optical system of a photocomposer machine, the physical mounting of the lens will establish the actual placement of the optical axis of that lens according to manufacturing tolerances which have been allowed in manufacturing the mount. Therefore, although engineering calculations can be made to place the font of characters, the lens and escapement, and the sensitive film holder all in proper relationship, the allowable tolerances may establish the optical axis somewhat differently than the calculation forecast. In a single lens machine, this deviation from forecast is not critical because everything in the composition will then take place from the composition base line as it actually is established.
Garth, U.S. Pat. No. 2,888,865, teaches the use of changeable lens systems for placing enlarged and/or reduced images in specific leading relationship for producing subscript and superscript members in relationship to a base line, the most common uses being mathematical formula and footnote indicia.
This prior patent uses detents to select a lens of proper magnification and base line relationship. The offset from the base line is deliberate, and the exactness is not critical.
However, when it is desired to change the lenses for the purpose of changing point size and maintaining the composition on the base line, particularly if the change is to occur within the line, the machine tolerances for a new lens positioned in the optical system will usually cause the base line to fall on a different line than the base line of the first lens. Hence, the alignment of a multiple lens machine, for example, that type which has a rotating turret of lenses, requires painstaking, skilled labor to adjust all lenses to a single base line. If time should cause any distortion of the mounting, the painstaking effort must be redone to reestablish the continuity of base line.